Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Gravity evidence

  1. Jun 4, 2010 #1
    Do we have evidence to support gravity is a wave (like the interference pattern for em wave)?

    Do we have evidence to support the gravity waves travel at the same speed of em waves?
  2. jcsd
  3. Jun 4, 2010 #2
    Gravity isn't thought to be a wave. Gravitational waves are to gravity as electromagnetic waves are to electromagnetism. A mass just gravitates, you need an accelerating mass to get radiation.

    There is evidence of gravitational radiation. In particular, some pulsar systems have large massive objects in close orbits, and we can accurately measure the orbital decay, and find it to be consistent with radiation of orbital energy as gravitational waves.

    Some low-confidence measurements. We're more confident that gravitational radiation travels no faster than c, for the same reasons nothing else can go faster than c, and there's no reason to expect it to go slower.

    Don't get impatient with the lack of detections. Gravitational radiation is expected to be hard to detect, and we're still inching toward instruments with the sensitivity we expect to be necessary.
  4. Jun 5, 2010 #3


    User Avatar
    Science Advisor

    Mathematically the speed c follows from the structure of Einstein equations (just as for the Maxwell equations).
  5. Jun 8, 2010 #4
    If you quantize a continuous field, "particles" pop out of the math. There are quasi-particles such as electron density surface waves. So gravity would behave this way on a very small scale, even if gravitons are not "real" in the same sense as electrons. They would still be "real" in the same sense as phonons!

    Yes and no. Gravity behaves exactly as predicted to very high precision (see Gravity Probe B, for example). If the speed of propagation of the disturbance was wrong, the details of frame-dragging and the gravito-magnetic effect would not pan out the same. Direct measurement of a gravitational wave (distortion of space-time) has never been made. The difficult part is is that the laws of GR conspire so that the direction of force is to where the object is now, not where you see it due to propagation delay. So navigating a spacecraft around Jupiter or Saturn to bounce around from moon to moon, with awesome precision, means that the understanding of gravity is pretty keen; however, the experience does not give a direct speed measurement of gravity (it seems to be instantaneous), but, the fact that it does seem that way means that the rules causing it are understood to high precision? Got it?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook